Microwave windows are used for passing energy between an evacuated section, such as an electron tube output and a gas-filled waveguide section. In the past, such windows have generally been made of a single piece of glass or ceramic sealed across the hollow cross section of the waveguide. See U.S. Pat. No. 3,255,377 and U.S. Pat. No. 3,096,462, both co-assigned with the present invention, for single piece windows of the prior art. Despite careful selection of materials with optimal thermal and mechanical characteristics, and the utilization of shapes and dimensions designed to minimize energy absorption, single piece windows are limited in their ability to handle high power transmission due to heat absorption.
One approach to increasing the power capacity of microwave windows has been to provide cooling means for absorbing the thermal energy imparted to the window. Examples of this approach are disclosed in U.S. Pat. No. 4,286,240 and U.S. Pat. No. 4,371,854, both co-assigned with the present invention. These patents disclose windows comprising two parallel window plates, narrowly separated, with means for circulating a cooling fluid between the plates.
While fluid cooled windows are now commonly employed in microwave tubes, removing heat from the window remains a problem with modern high power tube designs. A particular problem is related to the fact that microwave radiation patterns can cause localized heating or "hot spots" on the window surface beyond the capacity of the structure to provide cooling. When the radiation is of the circular electric field mode, these hot spots tend to occur in an annular band between the central and peripheral regions of the window.